Abstract The effects of exposure period and phosphine concentration on mortality of susceptible and resistant Sitophilus oryzae (L) were investigated. Although S oryzae is one of the world's most serious pests of stored grain there are few data on the practical significance of phosphine resistance in this species. The strains investigated were an Australian susceptible strain, a homozygous resistant strain exhibiting a level of resistance common in Australia and an unselected field strain from China with a much stronger resistance. Fumigations were carried out at 25 °C on adults and mixed‐age cultures. For adults of all three strains and mixed‐age cultures of the susceptible and resistant Australian strains, the relationship between concentration and time could be described by equations of the form C n t = k. In all cases n < 1, indicating that time was a more important variable than concentration. In all fumigations of adults the resistant strains were harder to kill than the susceptible strain. However, in fumigations of mixed‐age cultures, which contained the tolerant pupal stage, the difference between susceptible and resistant strains was more pronounced at lower concentrations than higher concentrations. For example, at 0.02 mg litre −1 the estimated LT 99.9 for mixed‐age cultures of the Australian resistant strain (27 days) is 3.4 times that of the susceptible strain (8 days), but at 1 mg litre −1 there is no difference between the two strains (4 days). Limited data on the Chinese resistant strain supported this finding. Twenty‐three days exposure at 0.02 mg litre −1 had no effect on mixed‐age cultures of this strain, but there were no survivors after 5 days exposure to 1 mg litre −1 . © 2002 Society of Chemical Industry